In recent years, there have been proposed force sensors including, arranged on a semiconductor substrate, resistance elements having the property of piezo resistance wherein the electric resistance varies in dependency upon a mechanical deformation to detect a force from changes in resistance values of the resistance elements. Further, acceleration sensors or magnetic sensors to which such force sensors are applied are also proposed. In either detector, a strain generative body partially having flexibility is used to detect a mechanical deformation produced in the strain generative body as changes in electric resistance of the resistance elements. A working body is provided for exerting a force on the strain generative body. If a weight body responsive to acceleration is used as the working body, an acceleration sensor is provided. Further, if a magnetic body responsive to magnetism is used as the working body, a magnetic sensor is provided.
For example, in U.S. Pat. No. 4905523, U.S. patent applications No. 295210, No. 362399, No. 470102, and No 559381, force/acceleration/magnetic sensors according to the invention of the inventor of this application are disclosed. Further, in U.S. patent application No. 526837, a novel manufacturing method for a sensor of this kind is disclosed.
The force sensors disclosed in these patent materials can detect a direction and a magnitude of an external force applied to a predetermined working point on the basis of changes in resistance values of resistance elements formed on a single crystal substrate. If a weight body is formed at the working point, it is also possible to detect, as a force, an acceleration exerted on the weight body. Accordingly, this permits application as an acceleration sensor. Further, if a magnetic body is formed at the working point, it is also possible to detect, as a force, magnetism exerted on the magnetic body. Accordingly, this permits application as a magnetic sensor.
However, the conventional force sensors (or acceleration sensors, or magnetic sensors based on the same principle) have the problem that there may occur interference in the output characteristic with respect to respective axial directions of two-dimensions or three-dimensions. For example, in the case of the three dimensional acceleration sensor, components in the X-axis, Y-axis and Z-axis directions of an acceleration exerted on a predetermined working point must be independently detected, respectively. In the case of conventional sensors, however, these components in respective axial directions interfere with each other. As a result, a detected value of the component in one axial direction was influenced to some extent by detected values of components in other axial directions. Such an interference is not preferable because it lowers reliability of measured values.
With the above in view, a first object of this invention is to provide a signal processing circuit capable of obtaining correct detected values free from interference of the components in other axial directions.
In order to mass produce such sensors to deliver them on the market, it is necessary to conduct a test or inspection at the final stage of the manufacturing process. The test for the force sensor can be carried out relatively with ease. Namely, this test may be accomplished by applying a force of a predetermined magnitude to the working point in a predetermined direction to check a detected output at this time. However, the test for the acceleration sensor or the magnetic sensor becomes more complicated. Since the sensor body is in a sealed state, it is necessary to check detected outputs while actually exerting, from the external, acceleration or magnetism thereon. Particularly, in the case of the acceleration sensor, it is the present state that a vibration generator is used to give vibration to the sensor body to carry out a test. This results in the problem that the testing device becomes large, and a test for a dynamic acceleration of vibration is only conducted.
With the above in view, a second object of this invention is to provide a testing method capable of more easily testing a sensor having a working body of force such as an acceleration sensor or a magnetic sensor, and to provide a sensor having a function capable of immediately carrying out this testing method.
Further, conventionally proposed sensors using resistance elements have a problem in the case of carrying out high sensitivity measurement. For example, in the case of the acceleration sensor, it is sufficient for the purpose of utilizing collision detection of a vehicle, etc. to have a function to detect acceleration of the order of 10 to 100G on the full scale. However, in order to detect a swing by hand of a camera, to conduct a suspension control for a vehicle, and to conduct a control for an antilock brake system for a vehicle, it is necessary to detect an acceleration of the order of 1 to 10G. For carrying out such a high sensitivity acceleration detection, it is necessary to increase the weight of a working body having a function to produce a force on the basis of an acceleration. However, in the case of the structure of conventional sensors, it was difficult to enlarge the working body.
In the case of the high sensitivity sensor, where a large force more than a predetermined limit is applied thereto, the danger that the semiconductor substrate may be damaged is increased. For this reason, it is necessary to provide, around the working body, a member for allowing displacement of the working body to limitatively fall within a predetermined range. This gives another problem that the structure becomes complicated.
Further, in the case of detecting force, acceleration, and magnetism, etc. exerted in three-dimensional directions, there would occur a difference between a detection sensitivity in a direction parallel to the surface of the semiconductor substrate and that in a direction perpendicular thereto. The fact that a sensitivity difference occurs in dependency upon the direction of detection is not particularly preferable for high sensitivity sensors.
With the above in view, a third object of this invention is to provide a sensor using resistance elements suitable for higher sensitivity physical quantity measurement, and a method of manufacturing such a sensor.